1. Field of the Invention
The present invention relates to an apparatus and a method for writing data onto a recording media, and more particularly, for writing data at high speeds onto a write-once recording media from which data will be read at high speeds.
2. Description of the Related Art
Over the last decade, there has been an ever-increasing demand for recording media having larger and larger amounts of storage capacity. One solution to this demand has been the so-called write-once recording media (WORM), such as optical disks. Data on an optical disk is stored in the form of a series of pits formed in the surface of the disk. WORMs are especially useful in applications where the data base is extremely large but relatively static, such as the Library of Congress book cataloging data or the U.S. Geological Survey's data of the topography of the United States, as WORMs can store in excess of two gigabytes of data on a single surface.
In view of the amount of data being stored on such media, efficient media management is essential. Media management systems for WORMs that have been introduced to date are relatively simple. The rotation rates of these WORMs during reading and writing is relatively slow, with the fastest of these WORMs being rotated at approximately eight revolutions per second (rps). Accordingly, devices that write data onto and read from such media function at relatively low data transfer rates and require media management techniques, need only be effective at these rates. However, computers are being introduced which process data at greater and greater rates. Personal computers are now being developed that will process data at many times the one to two million instructions per second (MIPS) rates now available in top-of-the line personal computers, and it is expected that computers that process data at 100 MIPS may soon become common. Storage media must be developed which can be read at sufficient speeds so as to satisfy the data transfer requirements of these faster computers.
When writing data onto WORMs, such as optical discs, at the relatively low speeds, it is actually possible for the writing device to watch and read the pits being formed by a laser in the surface of the WORMs. This is known as direct read during write (DRDW). Accordingly, during a write operation, a write controller can almost instantly determine whether the data written was the same as the data intended to have been written. However, present writing devices cannot watch the pits being formed at the reading and writing speeds required for the new faster computers. This loss of a valuable quality control tool is a major impediment to the development of higher speed WORMs.
Random access to the WORMs during the write process is a desirable feature. Data can be written on WORMs at predetermined addresses of the WORMs, which speeds up recall of data. Blank areas can be left to enable future updating of data. If the media management technique used for a WORM does not permit random access and changes to the database contained on the WORM are desired, the WORM must be replaced or supplemented. Supplementing a WORM requires a secondary storage media and some combination of additional software, additional hardware, and machine instructions to provide access for a user to the secondary storage media. These options are expensive and/or time consuming. Clearly, a media management technique which permits random access is desirable, as it would permit future updating and greater latitude when performing the initial write on a WORM. Thus, a need exists for a media management system which allows random access during a write operation to a high-speed WORM.
Any media management system must also take into account defects encountered during writing. Media defects may economically be determined only as the media is being written. Defects can be caused by gouges or other defects in the media surface itself, by dust which has accumulated on the media surface or which otherwise interferes with the laser beam during the write, or by vibrations during the write. Typically, less than 0.3 percent of addressable areas of a media surface will be declared defective. However, given the total number of addressable areas on a typical WORM, the number of defective areas will be large and poses a problem that must be addressed.
Efficiency is another important criteria for media management. The media management system must be designed so that an optimum average transfer rate performance is achieved, and a minimum spin up time is required for the WORM. In accessing data, the actual physical address is thousands of addresses away from its logical address as perceived by the host, an adequate system must be in place so that the data can be accessed quickly and efficiently. Otherwise, relatively large quantities of time are used to find the physical address. Additionally, given the demands for storage space outlined above, media utilization should be as efficient as possible.
One proposed method of media management is directed to dealing with media defects. When a defect is found in a first addressable area on the surface of the media during a write operation, a "slip" to a next addressable area of the media is performed. That is, when a defect is found in a first addressable area of the media, such as a surface defect or an incorrect write, the controller attempts to write the data meant for the first addressable area in the next addressable area. This system of media management has the highest media utilization efficiency, since the only media cost is for replacements. However, such a system is effective when a sequential write is being performed, and would be difficult to implement when random access to all sectors is desired. Given that the surface of a media may contain over two million physical addresses, and given the typical defect rate of 0.3 percent, the "physical" address of data being written can be thousands of addresses away from its "logical" address (the address to which the data was directed). Such differences would severely effect the data recovery efficiency in a random access system, as finding the required data would take an inordinate amount of time. However, after a read is started, the read access efficiency for one area in the portion to be read is very high, as the data will be found in the next sequential area of the WORM if the first area is defective (if the next area is not defective).
Another media management technique currently in use for the relatively slow WORMs preallocates portions of a WORM for replacement of defective areas. The replacement portions may be distributed over the WORM so as to provide fast access. When a defect is detected, a predetermined method is imposed by which the replacement portions of the WORM are utilized. Since the allocation of replacements can have no specific relationship to the distribution of defects, then some written relocation table must be utilized so that this variation can be tracked for reading. The maintenance of this table carries some impact on the media utilization efficiency. The area available for this maintenance should be chosen to match the maximal number of anticipated defects. The physical reference to each preallocated replacement is divided into two disjoint sets: the defect replacement set and the relocation table space.
Such a media management technique provides contiguous logical address space and random access to all sectors. However, media utilization cannot be 100%, since even if the area reserved for defects is equal to the area containing defects on a particular WORM, additional space is required on the WORM for mapping these replacements. Further, replacement access efficiency is low, as the defective (intended) area must be read, then the mapping area corresponding the defective area which indicates the location of the replacement area must be read, and finally the replacement area itself must be read. As these areas are not likely to be contiguous, several rotations of the WORM are likely to be necessary.
To date, no media management technique for high speed WORMs has been introduced which permits random access during both reading and writing, has high replacement access efficiency, and high media utilization efficiency.